As police question three men arrested in connection with the 7 July bombings, the BBC's security correspondent Gordon Corera says detectives have never given up in their hunt for anyone with links to the attacks.

Police have long emphasised that they have not stopped investigating the attacks of 7 July 2005 and that the investigation has been among the largest ever conducted by Scotland Yard.

In the year after the bombings, police collected nearly 30,000 exhibits, took 13,000 statements and worked through 6,000 hours of CCTV footage.

Until now no-one has been charged with any crime but the police have emphasised that they are still following a number of lines of inquiry.

A key aspect of the investigation was the work of MI5 and police in reconstructing the lives and movements of the bombers in the period leading up to the attacks, trying to work out who they met and talked to and whether any of these connections were significant.

Encouragement

This has involved using everything from telephone records to material seized from the men's houses during searches as well as other intelligence and information about who they interacted with in the local community.

The bombings themselves were remarkably cheap - estimates put the cost at about £8,000

The central question is whether anyone else might have been involved.

"We need to know who else, apart from the bombers, knew what they were planning," the police said in a statement. "Did anyone encourage them? Did anyone help them with money, or accommodation?"

The bombings themselves were remarkably cheap - estimates put the cost at about £8,000.

This means that there did not have to be any 'financier' behind the attacks, but police have long believed it is possible that others at the very least knew about the attacks and may have even provided material support in the form of transport or accommodation or other assistance.

Overseas links

However, proving this, especially when so much time has elapsed since the bombings, will be difficult, counter-terrorist officials concede.

The police have also said that some of their lines of inquiry are international rather than domestic. In practice, this means Pakistan.

Official reports have confirmed that two of the bombers Shehzad Tanweer and Mohammad Sidique Khan travelled together to Pakistan between November 2004 and February 2005.

Police inquiry a "painstaking task"

British intelligence agencies have long believed some form of operational training is likely to have taken place while Khan and Tanweer were in Pakistan together and that it is likely they did have contact with al-Qaeda figures.

It was on their return that the plot began to move ahead apace.

In the three months leading up to the bombing, the men were in contact with an individual or individuals in Pakistan who may have been giving them advice and direction.

Different approach

Pakistani authorities were passed at least 299 telephone numbers in their country which were either directly or indirectly linked to the bombing.

But identifying a specific mastermind or planner in Pakistan has proved difficult.

The working relationship between British investigators and their Pakistani counterparts is complex. There is co-operation but priorities and approaches are often different.

Police acknowledge that investigating 7/7 is a long painstaking task but they strongly believe that it's a task which should be continued for as long as it takes to ensure that anyone who was involved is brought to justice.